Quick and reliable chemical analysis of substances is a critical requirement in many industries. Many currently available analytical techniques involve an interaction of optical radiation with the sampled substance, where the molecules of the sample absorb at least some of the energy from the incident radiation. This energy can then be re-emitted through: diffuse reflectance (strong signal at the same wavelength, almost instantaneous); Raman scattering (very weak signal at a slightly shifted wavelength, almost instantaneous); and fluorescence (weak signal at a longer wavelength, time delayed, decaying with time).
Raman spectroscopy, in particular, is suitable for chemical analysis and monitoring. Raman frequency shifts are specific to the molecular vibrations. The presence of particular peaks in a Raman spectrum is indicative of particular molecular bonds and thus “fingerprints” a particular molecule. The intensities of Raman peaks are proportional to the chemical concentration of that molecule. Thus, Raman spectroscopy can be used to determine the sample composition both qualitatively, and, with proper calibration, quantitatively.
Many applications of chemical analysis involve aqueous samples. Raman spectroscopy is suitable for these applications, as water has very low absorption in the spectral region where most Raman shifts occur. Raman spectroscopy also does not require any particular sample preparation and can be used non-invasively (for example, through a vial). This makes it ideal for measurements in biomedical, environmental and process control applications, especially when combined with fiber optic sampling.
Optical fibers, including those based on fused silica glass, are useful for conducting spectroscopic measurements. Optical fibers transmit optical radiation efficiently over significant distances and enable remote measurements, for example, in hazardous environments by decoupling the measuring instrument from the sample to be measured. The design parameters of the optical fibers such as core diameter, numerical aperture and transmission range can be selected to best match the characteristics of both the sample and the instrument in use. Due to the size and flexibility of the optical fibers, measurements of small samples in confined spaces (such as in environmental monitoring, in-line process control or in-vivo biomedical applications) are possible. Optical fibers can be packaged with additional components such as lenses, filters, mirrors into application specific fiber optic probes.
In a typical Raman spectroscopy system comprising of spectrograph, fore optics, fiber bundles and remote fiber probe, it is usually either the spectrograph or the fiber bundle that determines the overall throughput (or “étendue”). As a result there is no advantage to collecting light which will not be later captured by the spectrograph. However, the design of the remote probe can be optimized to best match the capabilities of the spectrograph and the bundle.
Therefore, it is relevant to consider, amongst others, several factors in the design of fiber optic probes for remote spectroscopy: efficient collection of the back-scattered radiation (throughput); efficient rejection of the excitation radiation; the necessity to deal with spurious signals such as Rayleigh scattering and silica Raman coming from the fiber itself which often swamps the useful collected signal; and optimal coupling from the probe into the other parts of the system.
Fiber optic Raman designs in which one optical fiber delivered the excitation radiation to the sample and one or more collection fibers guided the collected signal to the analytical instrument are known. The collection fibers are arranged in parallel in one or more concentric rings surrounding the excitation fiber, resulting in low overlap between their respective emission and collection cones, with a zone of zero overlap close to the excitation fiber where the excitation radiation is the most intense. U.S. Pat. No. 4,573,761 (McLachlan) describes a design that decreases the zero overlap zone by tilting the fibers at the probe tip. U.S. Pat. No. 5,420,508 (O'Rourke) describes angle polishing, in order to improve the overlap and decreasing the zero overlap zone.
While compact and robust, these designs have several shortcomings. Since the excitation beam expands upon exiting the delivery fiber, the measurement area on the sample is fairly large, while the excitation radiation density is low. As a result the useful depth of focus of the probe is limited. Additionally, the collection fibers pick up the (useful) Raman signal, and also Rayleigh scattered excitation radiation which usually overwhelms the Raman signal. Furthermore, the interaction of the excitation radiation with fused silica generates a silica Raman signal which accumulates over the length of the fiber. For probes several meters in length, silica Raman became a major component of collected spectral data. These problems can be mitigated to some degree by using additional optical components in the probe to manipulate and filter both excitation and collection beams, thus improving the quality of collected signal. Generally, probes incorporating such additional optical components work by imaging the excitation fiber face to a spot on the sample and then re-imaging that spot onto the collection fibers, and are thus called imaging probes.
It is advantageous to perform filtering as close to the sample as possible. A conventional approach is to use a narrow laser band pass component close to the end of the excitation beam path and a rejection component placed between the sample and the collection beam path. The laser band pass ensures that only single frequency laser radiation is delivered to the sample, while the silica Raman signal as well as the Rayleigh scattered light from the fiber are either reflected back into the fiber or out of the system. After the interaction of the excitation radiation with the sample, the back-scattered signal contains both unshifted Rayleigh scattered excitation light and shifted Raman signal. The Rayleigh signal is much stronger (usually by several orders of magnitude) and should be prevented from entering the collection beam path. This can be accomplished with several types of components such as notch or long pass filters.
Several prior art designs are discussed in the following review articles'. F. Cooney, et. el., Appl. Specrosc. 50 (7), 836-848 (1996); T. F. Cooney, Appl. Spectrosc. 50 (7), 849-860 (1996); I. R. Lewis, and P. R. Griffiths, Appl. Spectrosc. 50 (10), 12A-30A (1996); and U. Utzinger, and R. R. Richards-Kortum, J. Biomed. Opt. 8, 121-147 (2003).
U.S. Pat. No. 5,112,127 (Carraba, et al.; see FIG. 1) teaches a design which incorporates optical components (a bandpass filter to clean up the excitation radiation, a dichroic filter to combine the two beam paths and a long pass filter to filter out silica Raman from collected signal) to selectively remove unwanted scattering from the collected signal. This design requires a very high performance dichroic filter which transmits a narrow band of excitation radiation to the sample and reflects a wide band of wavelengths into the collection path efficiently. Such components are difficult to manufacture and their performance varies with wavelength, affecting the relative strength of the Raman peaks observed. The physical layout of the probe makes it also fairly bulky, acceptable for industrial and lab use, but not practical for biomedical in-vivo work.
U.S. Pat. No. 5,377,004 (Owen et al.; FIG. 2) teach of a probe design with a collection beam path in-line with the sample, and with an excitation path folded into the main probe axis from the side. The beam combining element needs to be highly reflective over a narrow band and transmissive elsewhere. The probe employs holographic optical elements to filter and combine beams.
U.S. Pat. No. 5,615,673 (Berger, et al.) teaches a design capable of being used for biomedical applications where very low signals are observed. In order to improve collection efficiency, an additional parabolic component is placed in front of the probe which converts the collected radiation from a highly angular to almost parallel beam compatible with collection fiber acceptance angle. The tradeoff is that the collection bundle becomes larger and requires additional reformatting at the instrument input.
U.S. Pat. No. 5,953,477 (Wach, et al.) teaches of many techniques that can be applied to fiber optic Raman probes. In particular, it discloses a probe design in which the collection fibers are partially ground and coated with reflective layers in order to shape their collection cones away from the fiber axis and thus to improve the overlap between the excitation and collection volumes (while at the same time almost eliminating the dead space of zero overlap), resulting in a fivefold improvement in signal intensity over a beveled face probe design. The probe also incorporates small filtering elements coated directly onto the fibers to provide some of the advantages of filtered probes in a compact package. However, it is difficult to fabricate high performance filters on optical fiber faces with current technology. In addition, such filters perform sub-optimally as they are placed in converging light beams. Thus, these probes could not match the performance of imaging probes.
U.S. Pat. No. 6,038,363 (Slater, et al.; FIG. 3) discloses a probe with reduced background luminescence. This is achieved by introducing a transmissive combiner placed in the collection beam, with a small reflective aperture in the center which folded in the excitation beam into the probe optical path. Unlike the previous imaging probe designs, the beam combining is done not in amplitude (by filtering parts of the light away in expanded beams) but by wavefront (by blocking part of the probe aperture).
U.S. Pat. No. 7,647,092 (Motz, et al.; FIG. 4) discloses a non-imaging probe design with integral filtering, comprising a doughnut shaped long pass filter for collection, and a very small round band pass filter for excitation filtering. Beam steering is accomplished by adding a ball lens at the probe tip. The whole probe is under 2 mm in diameter and compatible for use with endoscopic medical applications.
In imaging probes described above, the overlapped excitation and collection beams are focused onto the sample using a common final lens element. This arrangement requires that the two beams are focused at the same distance from the probe, resulting in optimal overlap between excitation and collection volumes.
The overall throughput of these probes is often limited by either light gathering capability (characterized by the relative aperture, or f-number F/#) of the spectrograph used or by acceptance angle of the collection fibers (characterized by the numerical aperture, NA). In principle, for efficient coupling into the spectrograph, the acceptance cone of a collection fiber should be matched to the acceptance cone of the spectrograph. This has implications for the design of the probe itself, as the relative aperture of the collection optics should be matched to the fibers. The fastest commercial spectrographs have F/# of 1.8. Optical fibers with matching NA of 0.28 are available, but less common than the 0.22 NA fibers (equivalent to F/# of 2). Constructing an imaging system with such low F/# requires multi-lens assemblies. So, in conventional designs there is a trade-off between the volume illuminated by the excitation channel, the solid angle from which the same optics can collect the back-scattered signal and coupling that collected light into the spectrograph
In conventional imaging probe designs, the excitation and collection paths are routed in separate fibers, and the excitation and collection beams are expanded and overlapped within the probe body. To achieve their overlap, beam-combining components are employed, such as a dichroic filter, a narrow bandpass filter, a diffraction grating or a partial aperture mirror. Some loss of collected signal occurs in all the probe designs discussed above. Also, since the optical paths are overlapped, the excitation radiation is scattered back into the probe, increasing its background level.
An approach in which the two optical paths are isolated and independent is disclosed in U.S. Pat. Nos. 6,411,838 and 6,760,613 (Nordstrom et al.). This system uses a substantially coaxial and confocal configuration of emission and collection optical systems which are optically isolated. The illumination and detection systems are coaxial and arranged so that the excitation system forms a central obscuration within the illumination system, resulting in a central spot with no signal at the image plane of the illumination system.